Forbid adding non-users to Conpherence threads

Summary: Fixes T6724. Adds validation that participants are users.

Test Plan:

• Tried to add non-users, got an error.
• Added users normally.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6724

Differential Revision: https://secure.phabricator.com/D11955